Abstract:
The Global Oscillation Network Group ( GONG ) is a community-based project to conduct a detailed study of solar internal structure and dynamics using helioseismology. In order to exploit this new technique, GONG has developed a six-station network of extremely sensitive, and stable solar velocity imagers located around the Earth to obtain nearly continuous observations of the Sun's ``five-minute'' ... oscillations, or pulsations. GONG has also established a major, distributed data reduction and analysis system to facilitate the coordinated scientific investigation of these measurements.

Helioseismology utilizes waves that propagate throughout the Sun to measure, for the first time, the invisible internal structure and dynamics of a star. There are millions of distinct, resonating, sound waves, seen by the doppler shifting of light emitted at the Sun's surface. The periods of these waves depend on their propagation speeds and the depths of their resonant cavities, and the large number of resonant modes, with different cavities, allows us to construct extremely narrow probes of the temperature, chemical composition, and motions from just below the surface down to the very core of the Sun.

Without this ability to measure the structural properties and motions within the solar interior, our understanding of some of the most important processes in astrophysics would be limited to inferences from theory. This new direct evidence about the interior is providing far more stringent tests of stellar structure and evolution theory than that previously afforded by knowledge of just the global properties ( radius, luminosity, mass, composition and age ). Comparison of oscillation periods, predicted from solar models, with observations is expected to produce an accurate measurement of the helium abundance of the Sun; this will place bounds on cosmological models of the early universe, and may help clarify the solar neutrino deficit. At present, we can only guess at the nature of stellar convection. For stars like the Sun, essentially all of the energy released by fusion in the core is brought to the surface by convection currents whose large cells cannot be seen at the surface. However, these cells should perturb the oscillation periods in a distinctive way that will enable us to not only detect them but also to measure their properties. We similarly have no knowledge, from observations of flows at the solar surface, of how the rotation changes with depth and latitude in the interior, although rapid rotation would alter the evolutionary history of the Sun and similar stars. Rotation imparts a clear signature - a splitting - to the oscillation periods, and the first, albeit crude, measurements of the variation of this crucial stellar attribute throughout the interior convincingly demonstrated the power of helioseismology. We do not even have a firm grasp of the origins in the solar interior of the most easily seen of all solar features, sunspots, which are closely associated with solar activity and solar-terrestrial effects. Once again, helioseismology should detect the influence of these cool, magnetic islands in the otherwise horizontally homogeneous Sun.

A survey of the actual solar observing conditions, at various possible sites, was carried out to enable the selection of the locations that comprise the GONG network. Computer simulations using weather statistics, as well as estimates of equipment outages, from suitably located observatories indicated that a minimum of six sites, spaced roughly equally in longitude, would be required to achieve the design goal of a minimum of three years of nearly unbroken data. Fifteen different sites representing the six desired longitude bands were tested with small automatic sunshine monitors, and the sites that comprise the GONG network are:

The measured performance "fill" of the GONG site survey network was better than 93%. Current, real data, coverage is around 87%. This is less than the site survey fill because more stringent criteria is used for good data samples and semiannual preventive maintenance visits to each site bring down the instrument for about ten days per visit.

The five-minute oscillation is a subtle effect. Individual modes may exhibit velocities of less than 0.2 meters/second, while the sum of all of the modes is only a few hundred meters/second. The ultimate intention is to have the measurements be limited by the Sun's ``random'' surface motions. This means developing six stable instruments capable of making imaged velocity measurements with a precision of significantly less than one meter/second - one part in ten million! A low technological risk instrument based on a Michelson interferometer was selected, and it will be supported by a highly automated, portable installation, somewhat reminiscent of a spacecraft experiment in its design.

Each station in the network will produce more than 200 megabytes of data every day. Over the three year observing run, the raw data will exceed one terabyte, and the various processed data sets will exceed this several-fold. To keep up with the data flow, a pipe-line capable of roughly 6 Megaflops has be established to do the bulk processing and provide subsets of the data for the scientific community. Because of the widespread scientific participation, distributed data, software and analysis tools will be provided. Thus, in addition to a central facility, participating scientists will have access to readily transportable data archives and software, as well as shared analysis programs at their home institutions.

There are currently 130 individual members of the GONG, from 67 different institutions, and 20 nations, who are organized in Teams addressing specific scientific questions. They are active in the definition of the capabilities of the system as well as the development of the tools for the coordinated analysis of the results.

The deployment of the field instruments was completed in early October, 1995. The data acquisition will soon complete three years of operations. Currently, the project plans to continue operating the network for the remainder of the solar cycle.